Wood cleaning tools have been used that normally are carried by a comparatively long boom which is mounted on a forestry vehicle the boom having a parallel motion and being swingable around the attachment point on the vehicle. The arm can be moved outwardly from and inwardly towards the vehicle, the cleaning unit at the end of the boom being kept at a constant height above the ground due to the parallel motion of the boom. The constant height can normally be adjusted. A disc being rotatably driven by a motor has been used as cleaning unit, the disc being provided with knifes or flails hingedly attached to the periphery of the disc. The knives or flails will move at a comparatively high speed due to the large diameter of the disc in conjunction with the high rotational speed of the disc, normally about 1500 rpm. Since the knifes or the flailing tools further have to be relatively heavy in order to function in the desired way, their total kinetic energy will be high. If the flails strike objects like stones or pieces of wood, these objects therefore may be thrown a great distance from the unit at a high speed, since it is almost impossible to mount a protective device which entirely covers the rotating disc, the flails in that case not being able to reach the vegetation which is to be cleaned. This device thus requires a very large safety area with a radius which may be up to 100 meters. Since the vehicle naturally is located within the dangerous area, the unit must be provided with a protection on the side facing the driver, which means that the unit only can clean when it is moved sidewardly or outwardly from the machine, which is a disadvantage. A further disadvantage is that it is difficult to increase the rotational speed, and thus the efficiency of the unit, since the flails or knifes then may be subjected to forces being large enough to tear the flails or knifes away which, apart from impairing the efficiency of the unit, also would constitute a great danger. This known device thus has some disadvantages.
It has been tried to use a rotating blade instead of a rotating disc provided with flails or knifes in order to obtain a unit which does not have these disadvantages. Such a construction in practice eliminates the risk that objects are thrown outwardly from the unit, which means that the safety radius can be decreased to more normal values. In this case, the blade however must be protected against contact with larger stones and similar, since the blade will not be deflected when striking a fix object, in contrast to the hingedly attached flails or knifes. An object of the invention thus is to obtain a protective device for the blade. This is especially important when the blade is provided with a few, comparatively large serrations, since the entire unit, including the motor and the bearings, will be subjected to severe shocks when these serrations strike a fix object if the butting edge squarely strikes the object, with the consequence that the blade is abruptly stopped. An extreme form of such a blade would be a blade similar to the kind of blades used in ordinary rotary grass-movers having an elongated knife with only two cutting edges.